<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>点击右边字母右边滑动到对应城市</title>
  <style>
    body,div,h1,h2,ul,li,header {
      margin: 0;
      padding: 0;
    }
    ul,li {
     list-style: none;
    }
    html,body,.container {
      height: 100%;
    }
    .container {
      display: flex;
      flex-direction: column;
    }
    header {
      line-height: 60px;
      box-shadow: 1px 1px 10px #ddd;
      padding: 0 30px;
      color: #666;
    }
    .content {
      padding: 20px 50px;
      flex: 1;
      display: flex;
      overflow: hidden;
    }
    .city-box {
      flex: 1;
      overflow: hidden;
      display: flex;
      justify-content: space-between;
      border: 1px solid #efefef;
      padding: 20px 30px;
      font-size: 14px;
    }
    .city-list-hid {
      flex: 1;
      display: flex;
      overflow: hidden;
    }
    .city-list {
      flex: 1;
      min-height: 100px;
      overflow-y: scroll;
      padding-right: 30px;
      margin-right: -17px; /* 关键 */
    }
    .city-header {
      line-height: 30px;
      font-size: 18px;
      margin-bottom: 10px;
    }
    .city-body {
      display: flex;
      flex-wrap: wrap;
      margin-bottom: 20px;
    }
    .item-name {
      border: 1px solid #eee;
      border-radius: 15px;
      text-align: center;
      line-height: 26px;
      min-width: 68px;
      padding: 0 10px;
      color: #666;
      margin: 0 10px 10px 0;
      cursor: pointer;
      transition: 300ms;
    }
    .item-name:hover {
      background: green;
      color: #fff;
    }
    .city-list-menu {
      font-size: 12px;
      line-height: 16px;
      width: 20px;
      color: #999;
      align-self: center;
      text-align: center;
    }
    .city-list-menu-li {
      transition: 300ms;
      cursor: pointer;
    }
    .city-list-menu-li:hover, .city-list-menu-li.active {
      font-size: 16px;
      color: green;
    }
  </style>
</head>
<body>
  <div class="container">
    <header><strong>城市对应字母功能：</strong>点击右边字母右边滑动到对应城市，滚动左边城市右边字母自动选中</header>
    <div class="content">
      <div class="city-box" id="cityBox">
        <div class="city-list-hid">
          <div class="city-list" id="cityList">
          </div>
        </div> 
        <div class="city-list-menu">
          <ul id="cityListMenuUl">
          </ul>
        </div>
      </div>
    </div>
  </div>
  <!-- 为了快速构造结构 这里引入jquery， 其实核心代码可以不用 jquery -->
  <script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>
  <script>
    /* 定义一些变量 */
    // 城市数据数组
    var cityGroup = [];
    var cityBoxOffsetTop = 0;
    // 每个字母距离顶部的距离
    var offsetTops = [];
    // 字母标题的高度
    var threshold = 30;
    // 获取城市对应字母
    $.getJSON('./city-first-char.json', function(res) {
      cityGroup = res.cityGroup;
      initCityHtml();
    });
    // 初始化结构
    function initCityHtml() {
      var cityList = '';
      var cityListMenuUl = '';
      cityGroup.map(function(e, i){
        cityListMenuUl += `<li class="city-list-menu-li ${i===0?'active':''}" onclick="positionIndex('${e.firstChar}', this)" data-id="${e.firstChar}">${e.firstChar}</li>`;
        var cityListBody = '';
        e.cityList.map(function(city){
          cityListBody += `
            <div class="item-name">${city.name}</div>
          `
        });
        cityList += `
          <div class="item">
            <div class="city-header" id="city-${e.firstChar}">${e.firstChar}</div>
            <div class="city-body border-btm">
            ${cityListBody}
            </div>
          </div>
        `;
      });
      $('#cityList').html(cityList);
      $('#cityListMenuUl').html(cityListMenuUl);
      // 关键
      getTopData();
    }
    // 获取距离顶部高度 关键函数 纯 js 实现
    function getTopData() {
      cityBoxOffsetTop = document.getElementById('cityBox').offsetTop;
      console.log(cityBoxOffsetTop);
      cityGroup.forEach((item, index) => {
        let offsetTop = document.getElementById('city-' + item.firstChar).offsetTop;
        offsetTops.push(offsetTop-cityBoxOffsetTop);
      });
    }
    // 点击字母滑动到对应城市 关键函数 纯 js 实现
    function positionIndex(index, that) {
      document.querySelectorAll('.city-list-menu-li').forEach(function(e){e.classList.remove('active')});
      that.classList.add('active');
      let offsetTop = document.querySelector('#city-' + index).offsetTop;
      document.getElementById('cityList').scrollTop = offsetTop-cityBoxOffsetTop-threshold;
    }
    // 滚动条滚动 关键函数 纯 js 实现
    document.getElementById('cityList').addEventListener('scroll', function(event){
      document.querySelectorAll('.city-list-menu-li').forEach(function(e){e.classList.remove('active')});
      if(event.target.scrollTop > threshold) {
        offsetTops.every((item, index) => {
          if(event.target.scrollTop <= item) {
            document.querySelector(`.city-list-menu-li[data-id=${cityGroup[index].firstChar}]`).classList.add('active');
            return false;
          }
          return true;
        });
      }else {
        document.querySelector(`.city-list-menu-li[data-id=A]`).classList.add('active');
      }
    })
  </script>
</body>
</html>